SANFORD - A judge tossed out a Florida police detective's statement that he found George Zimmerman credible in his description of fighting with Trayvon Martin, a decision that could benefit prosecutors who are trying to discredit the defendant's self-defense claims.

Other efforts by prosecutors to attack Zimmerman's version of events on Tuesday included the cross examination of a friend he called after shooting Martin and the testimony of a doctor who found the defendant's injuries to be insignificant. They also sought to introduce school records that indicate the neighborhood watch volunteer had studied the state's self-defense law, in another swipe at his truthfulness.

Prosecutors took the unusual step of trying to pick apart the statements of a Sanford, Fla., police investigator they'd called as a prosecution witness because some of what he said appeared to help the defense. Prosecutor Bernie de la Rionda asked the judge to strike Detective Chris Serino's statement that he thought Zimmerman was credible when he described how he got into a fight with Martin. Serino was the lead investigator on the case.

The initial lack of an arrest by the Sanford police following the death of Martin, an unarmed black teenager, in February 2012 led to protests across the nation and spurred a debate about race and the laws of self-defense. Zimmerman's father is white and his mother is from Peru. Sanford's police chief was fired months later, and a Florida special prosecutor charged Zimmerman with second-degree murder.

Zimmerman has said he fatally shot Martin, 17, in self-defense because he was banging his head into a concrete sidewalk. Zimmerman, 29, could get life in prison if convicted of second-degree murder. He has pleaded not guilty.

To earn a conviction on the charge, prosecutors must prove there was ill will, spite or a depraved mind by the defendant.

The prosecutor also questioned Serino about his opinion that Zimmerman didn't display those negative emotions toward Martin.

De la Rionda played back Zimmerman's call to police to report the teen walking through his gated community. Zimmerman uses an expletive, refers to "punks" and then says, "These a-------. They always get away."

The detective conceded that Zimmerman's choice of words could be interpreted as being spiteful.

The state has argued that Zimmerman profiled Martin from his truck and called a police dispatch number before he and the teenager got into a fight. Zimmerman has denied the confrontation had anything to do with race, as Martin's family and their supporters have claimed.

Several moves by prosecutors Tuesday were aimed at showing inconsistencies in Zimmerman's statements.

Prosecutors asked the judge to allow them to introduce school records showing Zimmerman took a class that addressed Florida's self-defense law. They say it will show he had knowledge of the law, even though he claimed he didn't during an interview with Fox News talk show host Sean Hannity. The interview was played for jurors.

O'Mara objected, saying the school records were irrelevant. He referred to the prosecution's efforts to introduce them as "a witch hunt."

The judge said she would rule later in the week.

O'Mara told CNN on Tuesday night that if prosecutors start bringing up Zimmerman's past, "then it really brings in what Trayvon Martin brings to the table, all of his violent acts that we know about and some of the fighting that he was involved in."

Late Tuesday morning, the prosecution questioned Mark Osterman, a Zimmerman friend who spoke with him after the shooting and later wrote a book about the case.

Under questioning by de la Rionda, Osterman said that Zimmerman told him Martin had grabbed his gun during their struggle, but that Zimmerman was able to pull it away.

That account is different from what Zimmerman told investigators in multiple interviews. In those interviews, he only said it appeared Martin was reaching for his gun prior to the shooting. He never told police the teen grabbed it.

"I thought he had said he grabbed the gun," Osterman said. "I believe he said he grabbed the gun."

A Sanford Police Department fingerprint examiner testified that none of Martin's prints were found on the gun.

Prosecutors also called a medical examiner who had reviewed evidence for them to the witness stand. Dr. Valerie Rao testified that Zimmerman's injuries were insignificant, bolstering the prosecution's claims that Zimmerman's life wasn't in jeopardy during his fight with Martin. Rao was not the medical examiner who performed the autopsy on Martin.

"They were so minor that the individual who treated and examined Mr. Zimmerman decided stitches weren't required," Rao said.

O'Mara told CNN that he believed prosecutors may finish presenting their case Wednesday or Friday, and that defense attorneys will be calling witnesses most of next week.